Friday, September 24, 2004

IndexMap

In my project there are several idiomatic uses of JDK collections. One is using Map to uniquely index domain objects by some field. The code is simple:

public class IndexMap extends SafeHashMap {
    public static interface Mapper {
        public Object getKeyFor(final Object value);
    }

    private final Mapper mapper;

    public IndexMap(final Class keyClass, final Class valueClass,
            final Mapper mapper) {
        super(keyClass, valueClass);

        this.mapper = mapper;
    }

    public IndexMap(final Class keyClass, final Class valueClass,
            final Mapper mapper, final Collection values) {
        this(keyClass, valueClass, mapper);

        addAll(values);
    }

    public void add(final Object value) {
        put(mapper.getKeyFor(value), value);
    }

    public void addAll(final Collection values) {
        for (final Iterator it = values.iterator(); it.hasNext();)
            add(it.next());
    }
}

(SafeHashMap is another JDK collection extension. It forbids null keys and values, and requires they be of certain classes.)

Idiomatic use looks like:

new IndexMap(KeyType.class, DomainType.class, new IndexMap.Mapper() {
    public Object getKeyFor(final Object value) {
        return ((DomainType) value).getKey();
    }
}, initialValues);

Which indexes a collection of DomainType domain objects by the key property.

UPDATE: Because of editing several files at once, I suffered a brain fart and mixed IndexMap (the point of this post) with AutoHashMap (another, still interesting collection).

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